Complete AV block question

[sunshine02]

In UWorld, there's a question that in short, goes like this: there's complete desynchronization between p waves and qrs complexes. EKG demonstrates bradycardia with narrow QRS complexes and regular rhythm. What's responsible for pacing the ventricles?

Since it says complete desynchronization between p waves and qrs complexes, that should mean complete 3rd degree AV block right? Then that means the site responsible for pacing the ventricles is below the AV node since in 3rd degree block, NO impulse is traveling from AV node to ventricles.

However, the answer is AV node. I'm confused as to why. Can someone explain why the answer is AV node?

---

Members don't see this ad.

 

[wanderingorion]

The AV node has its own intrinsic rhythm (called the AV nodal rhythm). Usually, the SA node (which is faster) drowns it out, but it takes over when there is heart block. 3rd degree block means that the SA node signal is not propagated to the ventricles, but not that there is no impulse from the AV node to the ventricles.

 

[Ven0m]

Yeah +1 on orion, complete AV block = 3rd degree AV block

 

[sunshine02]

Thanks for your guys' help! However, isn't 3rd degree AV block when no action potentials go through the AV node and ventricles never see action potentials from the atria? See this website below: Third-Degree Atrioventricular (AV) Block ECG Review - Criteria and Examples | LearntheHeart.com

Multiple other sites say the same. I get that SA node isn't propagated to the ventricles but impulse from AV node shouldn't be going to the ventricles either, right? Otherwise it's not a complete AV block?

 

[NecrotizingFasciitis]

I think the website should have worded it more specifically, like this: in 3rd degree block, there are no APs conducted through the AV node from the SA node.

I think the context is implied in the website you linked. But the AV node definitely beats on it's own once it realizes the SA node AP's aren't coming anymore.

Kinda like how some parents make their kids fend for themselves at age 18 ya know. Like hey son (SA node to AV node), I know we've provided you with money (action potentials) for food/clothes (contractions) these past 18 years, but now you're completely on your own (3rd degree AV block.) And so the son (AV node) goes off & gets a job to make his own money (automaticity; action potentials), but the job isn't as high paying (slower rate) as his parents' job, so his life isn't as high quality (HR) as his parents' w/ respect to buying nice things.

 

[Gurby]

I suppose you would call this a 3rd degree block with junctional escape rhythm. Note the rate in the 40-60's as another clue that this is coming from the AV node.